muscle tissue

Muscle Tissue Overview

• Muscle tissue is specialized for contraction, producing voluntary or involuntary body part movement.
• It is one of the basic tissues in the body.
• Muscle cells (fibers/myocytes) are elongated and narrow.
• "Sarco" is Greek for muscle; "Myo" is Latin for muscle. These prefixes are used in describing muscle.

Properties of Muscle Tissue

• Excitability: Responds to nerve signals by generating electrical signals.
• Contractility: Shortens and generates force.
• Extensibility: Can be stretched.
• Elasticity: Returns to its original shape.

Functions of Muscle Tissue

• Movement.
• Maintaining posture.
• Stabilizing joints.
• Thermogenesis (transforming chemical energy to mechanical energy, thus creating force).
• Increasing muscle mass, boosting the immune system.

Types of Muscle Tissue

• Skeletal muscle
• Smooth muscle
• Cardiac muscle

Skeletal Muscle

• Connects to bones via tendons, enabling a wide range of movements and functions.
• It is the most common and abundant muscle type.
• It is striated and under voluntary control.
• Examples include muscles in the limbs and body wall.

Characteristics of Skeletal Muscle

• Microscopic: Long cylindrical fibers (muscle cells/myocytes), cross-striations, multiple nuclei (peripherally distributed), and unbranched.
• Location: Attached to bones via tendons.
• Nervous Control: Voluntary, supplied by somatic nerves, governed by the somatic nervous system.
• Speed of Contraction: Fast.
• Size: Very large (10-100 µm diameter, up to 35 cm length).
• Connective Tissue Components: Endomysium, perimysium, epimysium.
• Contractile Proteins: Organized into sarcomeres.
• Junctions/Rhythmicity: No junctions between muscle fibers, no auto-rhythmicity.
• Regulator Proteins: Troponin and Tropomyosin; regulate contraction.

Parts of a Muscle

• Belly: The fleshy part of the muscle.
• Tendons: Fibrous parts, tightly connected to the bones or cartilage. More collagen fibers, less vascular, non-elastic.
• Aponeurosis: Thin fibrous connective tissue sheets that connect muscles to bones and provide stability and strength, especially in flat muscles like abdominal aponeurosis.

Attachments of Muscles

• Origin: Relatively fixed attachment point during muscle contraction; generally located proximally (closer to the center of the body).
• Insertion: Attachment that moves during muscle contraction; generally located distally (further from the center of the body).

Muscle Organization (Arrangement of Muscle Fibers)

• Parallel Muscles: Muscle fibers run parallel to the muscle's length. This is the most common type in the skeletal muscles. Examples of different parallel muscles include: fusiform, strap, and quadrilateral muscles, including the biceps brachii, sartorius, and pronator quadratus.

• Convergent Muscles: Muscle fibers spread out like a fan at one end and converge to a single point at the other, like the pectoralis major.

• Pennate Muscles: Muscle fibers are arranged at an angle to the tendon like a feather. Include different types: unipennate (e.g., palmar interossei), bipennate (e.g., dorsal interossei) and multipennate (e.g., deltoid).

• Circular Muscles: Muscle fibers arranged concentrically around an opening, such as the orbicularis oculi and orbicularis oris.

Nomenclature of Muscles

• Shape, Size, Origin, Number of Heads, Location.

Actions of Skeletal Muscles

• Agonist (Prime Mover): Muscle primarily responsible for a specific movement; directly brings about the intended movement.
• Antagonist: Muscle opposing the action of the agonist.
• Synergist: Muscle assisting the agonist to perform its action, stabilizing the joint during the movement.
• Fixator: Stabilizes proximal joints to allow distal joint movement; a body part stabilizer during a movement of another part; example includes rotator cuff muscles during biceps curl.

Skeletal Muscle Structure

• Surrounded by epimysium (connective tissue).
• Composed of bundles of muscle fibers called fascicles, encased by perimysium.
• Individual muscle fibers have endomysium, a connective tissue sheath.

Skeletal Muscle Fiber Structure

• Contains myofibrils, bundles of contractile proteins.
• Myofibrils are composed of numerous sarcomeres, functional units of contraction.
• Sarcomeres are divided by Z lines.
• Myofilaments: Actin (thin) and Myosin (thick), making up sarcomeres.
• Striations: The overlap pattern of thick and thin filaments creates the striped appearance of skeletal muscles.
• T tubules: Membrane invaginations.
• Sarcolemma: Plasma membrane of the muscle fiber.
• Sarcoplasmic reticulum: Smooth endoplasmic reticulum that stores calcium.

Smooth Muscle

• Involuntary, non-striated muscle, fusiform shape, single, centrally located nucleus.
• Located in the skin (arrector pili muscles), reproductive/respiratory/urinary tracts, hollow organs (e.g., intestines, bladder, uterus, stomach), vessels (blood vessels), and eyes (iris contraction/dilation and lens movement).

Characteristics of Smooth Muscle

• Elongated, spindle-shaped cells.
• Centrally located nucleus.
• Absence of striations, myofibrils, sarcomeres, and T tubules.
• Supplied by autonomic nerves, involuntary.
• Small size (3-8 µm diameter).
• Only endomysium connective tissue component.
• Gap junctions in visceral smooth muscle.
• Auto-rhythmicity of visceral smooth muscle.
• Slow speed of contraction.

Smooth Muscle Structure

• Filaments (thick myosin, thin actin).
• Dense bodies (similar to Z disks in striated muscle), which link filaments and transmits the contractile forces.

Cardiac Muscle

• Involuntary, striated muscle.
• Exclusively in the heart (myocardium).
• Muscle fibers branch and interconnect, forming a network.
• Muscle fibers are short, branched, cylindrical.
• Cells (cardiomyocytes) connected by intercalated disks (allow communication and rapid signal transmission)
• Contains large mitochondria.
• Contains prominent transverse striations.
• Moderate speed of contraction.
• Contains troponin and tropomyosin regulator proteins.

Diseases of the Muscle Tissue

• Myasthenia Gravis: Neuromuscular disorder; weakness in skeletal muscles, caused by communication impairment between nerve cells and muscles. Autoimmune; antibodies attack components of the postsynaptic membrane, impairing neuromuscular transmission.
• Duchenne Muscular Dystrophy: Progressive weakening and loss of skeletal and heart muscle, genetic disorder arising from a mutation in the dystrophin gene.